February 12, 2014

Juggling Reveals Clues On Role Of Sensory Feedback In Running

The act of juggling is shedding new light on the role our vision and sense of touch have in helping control the way we engage in rhythmic movements such as running, researchers from John Hopkins University reported in the Journal of Neurophysiology.

The study’s findings could someday aid in the treatment of people with neurological diseases, and help improve prosthetic limbs, the researchers said.

The Johns Hopkins engineers, led by Noah Cowan, associate professor of mechanical engineering, noted that juggling involves repeated rhythmic motions to keep multiple balls aloft. These motions are similar to forms of rhythmic movement commonly seen in the animal world, where effective locomotion is critical.

“It turns out that the art of juggling provides an interesting window into many of the same questions that you try to answer when you study forms of locomotion, such as walking or running,” said Cowan, an amateur juggler since he was a teenager.

“In our study, we had participants stand still and use their hands in a rhythmic way. It’s very much like watching them move their feet as they run. But we used juggling as a model for rhythmic motor coordination because it’s a simpler system to study.”

Cowan and his team wanted to examine how the brain uses vision and the sense of touch to control this type of behavior, so they set up a simple virtual juggling scenario. Study participants held a physical paddle connected to a computer, and were instructed to bounce an on-screen ball repeatedly up to a target area between two lines, also drawn on the monitor.

In some trials, the participants had only their vision to guide them, while in others they also received a brief impulse on their real-world paddle whenever the digital ball hit the onscreen paddle. This mimicked the sensation a participant would feel if a real ball had actually struck the paddle they were holding.

With the added touch sensation, or haptic feedback, the participants made about half as many errors in the task, the study revealed.

“We have a pretty good understanding as to why,” said Cowan.

“One of the tricky challenges in juggling is catching a rhythm; that is, getting yourself entrained with the movement of the ball. It’s about timing your own action with the action in the environment. When you get the pulse of haptic feedback at the exact moment the ball hits the paddle, it give you a precise sense of the timing for the juggling pattern that you’re trying to achieve.”

M. Mert Ankarali, a Johns Hopkins mechanical engineering doctoral student who was lead author of the study, said our sense of touch provides us with important timing cues.

“The human nervous system gets feedback all of the time from our sense of vision. But the important thing about the sense of touch while juggling is that we get a precise timing cue that complements the continuous visual feedback. This timing cue is very important for us to get the rhythm of the juggling task.”

Although adding the touch feedback did not appear to improve the participants’ ability to correct for any juggling errors made while trying to hit the ball into the target zone, it did enable the participants to make fewer errors overall.

“The haptic sensation is just a tiny bit of feedback that’s provided once per juggling cycle,” Cowan explained.

“Yet that tiny bit of information seems to be critical for people to improve their juggling performance. We think that’s because while vision provides excellent spatial and positioning information, the haptic information provides very important timing information.”

When humans and animals walk or run, their sense of touch plays a vital role, Cowan said. As the runner’s feet touch the ground, they alert the nervous system to adjust the movement of the legs to accommodate changes in the running surface. The brain’s ability to instantly integrate information coming from both the eyes and the sense of touch is a critical part of successful running, juggling and other repetitive movements, he said.

The researchers said that future studies of the connection between sensory feedback, timing and limb movements could improve understanding about how some neurological diseases might disrupt the brain’s timing of movements by arms and legs. Future findings may also assist engineers in making better touch-sensitive artificial limbs and robots.